1. Field of the Invention
This invention relates to a method and apparatus for burning fuel resulting in low NO.sub.x formation. More specifically, this invention relates to a staged fuel and air injection burner.
2. Description of the Prior Art
With the advent of contemporary environmental emission standards being imposed by various governmental authorities and agencies involving ever stricter regulations, methods and apparatus to suppress the formation of oxides of nitrogen during combustion with air are becoming increasingly numerous. Various techniques have been suggested and employed in the design and operation of burners and furnaces to meet those regulations. Thus it is known that to burn a hydrocarbon fuel in less than a stoichiometric concentration of oxygen intentionally produces a reducing environment of CO and H.sub.2. This concept is utilized in a staged air type low NO.sub.x burner wherein the fuel is first burned in a deficiency of air in one zone producing a reduced environment that suppresses NO.sub.x formation and then the remaining portion of the air is added in a subsequent zone. Staged fuel has also been suggested wherein all of the air and some of the fuel is burned in the first zone and then the remaining fuel is added in the second zone. The presence of an over abundance of air in the first reaction zone acts as a diluent thus lowering the temperature and suppressing formation of NO.sub. x. It has also been proposed to recirculate fule gas to accomplish the lowering of the flame temperature.
However, each of the prior art processes have certain inherent deficiencies and associated problems which have led to limited commercial acceptance. For example, when burning fuel in a substoichiometric oxygen environment the tendency for soot formation is increased. The presence of even small amounts of soot will alter the heat transfer properties of the furnace and heat exchanger surfaces downstream from the burner. Also, flame stability can become a critical factor when operating a burner at significantly sub-stoichiometric conditions.